Bimorph piezoelectric device for acceleration sensor and method of its manufacture

ABSTRACT

The present invention relates to a bimorph type piezoelectric acceleration sensor used in detecting vibrations of a variety of equipment such as a hard disk, CD-ROM and the like and has the objective of providing a bimorph type piezoelectric element for acceleration sensor of a small size, high sensitivity, a narrow range of sensitivity variation and a low cost by forming a freely vibrating part and a supporting means at the same time in single-piece construction through a grinding away process applied to lithium niobate. One section of a structure formed of two of a lithium niobate single-crystal plate directly bonded together with the polarization directions thereof reversed each other is applied with a grinding away process to produce a freely vibrating part ( 32   a ) and the remaining section, where no grinding away process has been applied, serves as a supporting member ( 33   a ), thus both being formed simultaneously and made integral with each other, and electrodes ( 31   a,    31   b ) are formed by electroless plating.

FIELD OF THE INVENTION

[0001] The present invention relates to a bimorph type piezoelectricelement for acceleration sensor that is used in vibration detection ofvarious mechanisms such as a hard disk, CD-ROM and the like andparticularly relates to a piezoelectric element for acceleration sensorand its manufacturing method.

BACKGROUND OF THE INVENTION

[0002] As the portable type personal computers come into wide use, ashock-proof capability of a hard disk memory device (referred to as HDDhereafter) is considered important. Many methods have been put to theactual use to detect mechanical shocks as acceleration and suchdetecting means are required to be of a small and thin surface mounttype that can be built into the HDD. An acceleration sensor employingpiezoelectric ceramic is widely used to satisfy the above requirement.The reason why piezoelectric ceramic can be used as an accelerationsensor is that an applied force F produced in proportion to theacceleration (mechanical shock) ax causes distortion to occur in thepiezoelectric ceramic and the distortion can be taken out as electricalcharge (voltage). This can be expressed by an equation as follows:

F=k1×α  (1)

Q(V)=k2×F  (2)

[0003] , where k1 and k2 are constants.

[0004]FIG. 12 shows structural examples of the acceleration sensors thatuse piezoelectric ceramic. FIG. 12(a) shows a bimorph type accelerationsensor of one end supported beam (cantilever) structure and FIG. 12(b)shows a bimorph type acceleration sensor of two end supported beamstructure. FIG. 13 shows a method for manufacturing the bimorph typeacceleration sensor of cantilever structure of FIG. 12(a) and FIG. 14shows a method for manufacturing the bimorph type acceleration sensor oftwo end supported beam structure.

[0005] In the foregoing drawings, the reference symbols 1 a to 1 dindicate piezoelectric ceramic, the reference symbols 2 a to 2 dindicate electrodes formed on the piezoelectric ceramic, the referencesymbols 7 h and 7 j indicate bimorph type piezoelectric elements, thereference symbols 3 a, 3 c and 3 d indicate adhesives to bond thebimorph type piezoelectric ceramic and the reference symbols 4 a, 4 cand 4 d indicate supporting members to support and fix the bimorph typepiezoelectric elements, respectively. In the cantilever structure ofFIG. 12(a), a section L1 of the bimorph type piezoelectric element 7 h,the section L1 not being fixed to the supporting member 4 a, forms afreely vibrating part for acceleration detection, in which distortionoccurs in proportion to acceleration and electric charge is generatedaccording to the extent of distortion, and the electric charge thusgenerated is detected as indicating the magnitude of acceleration. Inthe two end supported beam structure of FIG. 12(b) also, a section L2 ofthe bimorph type piezoelectric element 7 j, the section L2 not beingfixed to the supporting members 4 c and 4 d, forms a freely vibratingpart for acceleration detection in the same way as in FIG. 12(a).

[0006] The method for manufacturing these acceleration sensors are as inthe following:

[0007] Pairs of piezoelectric ceramic 1 a to 1 d, in each respectivepair of which polarization is reversed between the opposed pieces of thepiezoelectric ceramic, are put together by an adhesive or in the stateof green sheet and fired in a single-piece construction, therebyproducing bimorph type piezo-electric elements 7 h and 7 j, which arethen attached with supporting members 4 a, 4 c and 4 d by the use ofadhesives 3 a, 3 c and 3 d for fixing, respectively, to complete acantilever structure or a two end supported beam structure.

[0008] However, the prior art method of fixing a piezoelectric elementto a supporting member or supporting members by adhesion tends to causea freely vibrating part of the piezoelectric element to exhibitdimensional variations, resulting in not a constant state of anchorage,thereby bringing about the problem of variation in sensitivity againstacceleration. When acceleration α is applied to a bimorph typepiezoelectric element having a freely vibrating part of a length L, theelectrical charge (voltage) Q (V) generated in the bimorph typepiezoelectric element is derived from the following equation:

Q(V)=k3×L ²×α  (3)

[0009] , where k3 is a constant.

[0010] Since the generated electrical charge represents the sensor'ssensitivity, the equation (3) tells that the sensor's sentivity isproportionate to the square of the length L of the freely vibratingpart.

[0011] Furthermore, the prior art manufacturing method employs a methodof bonding each respective piezoelectric element to a supporting memberby an adhesive, thereby having been making a cost reduction difficult torealize.

DISCLOSURE OF THE INVENTION

[0012] The present invention deals with the problems as described in theabove with the objective of providing a bimorph type piezoelectricelement for acceleration sensor that is small in size, high insensitivity and reduced in sensitivity variations, and the manufacturingmethod thereof.

[0013] In order to achieve the foregoing objective, the bimorph typepiezo-electric element for acceleration sensor of the present inventionhas a bimorph type piezoelectric element for acceleration sensor formedof a pair of piezoelectric single-crystal plates polarized in differentdirections between the two piezoelectric single-crystal plates and puttogether face to face by direct bonding, and characterized by having afreely vibrating part formed on at least one of the foregoing pair ofpiezoelectric single-crystal plates by grinding away partially and alsoa supporting member or supporting members formed of the part that is notapplied with a grinding away process and made integral with theforegoing freely vibrating part at one end or both ends thereof, and themanufacturing method includes:

[0014] a first step of direct bonding, whereby the foregoing pair ofpiezoelectric single-crystal plates are bonded face to face and heated;

[0015] a second step of forming a freely vibrating part by grinding awayat least one of the directly bonded pair of piezoelectric single-crystalplates in the direction of raows or columns at a predetermined spacingto a predetermined depth;

[0016] a third step of forming electrodes on the ground away mainsurface of the foregoing piezoelectric single-crystal plate; and

[0017] a fourth step of forming supporting members by cutting both theground away freely vibrating part and the part that is not applied witha grinding away process at a predetermined spacing in the directions ofrows and columns.

[0018] According to this invention, a bimorph type piezoelectric elementfor acceleration sensor that is small in size, high in sensitivity andreduced in variations in sensitivity can be produced at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1(a) is a cross-sectional view of a bimorph typepiezoelectric element for acceleration sensor in a first exemplaryembodiment of the present invention and FIG. 1(b) is a perspective viewof the bimorph type piezoelectric element of FIG. 1(a).

[0020]FIG. 2(a) is a cross-sectional view of a bimorph typepiezoelectric element for acceleration sensor in a second exemplaryembodiment of the present invention and FIG. 2(b) is a perspective viewof the bimorph type piezoelectric element of FIG. 2(a).

[0021]FIG. 3 shows perspective process views of a method formanufacturing a bimorph type piezoelectric element for accelerationsensor in a third exemplary embodiment of the present invention,

[0022]FIG. 4 is a continuation of FIG. 3 and

[0023]FIG. 5 shows cross-sectional views of the manu-facturing steps ofFIG. 3 and FIG. 4.

[0024]FIG. 6 shows perspective process views of a method formanufacturing a bimorph type piezoelectric element for accelerationsensor in a fourth exemplary embodiment of the present invention,

[0025]FIG. 7 is a continuation of FIG. 6 and

[0026]FIG. 8 shows cross-sectional views of the manufacturing processesof FIG. 6 and FIG. 7.

[0027]FIG. 9 shows perspective process views of a method formanufacturing a bimorph type piezoelectric element for accelerationsensor in a fifth exemplary embodiment of the present invention,

[0028]FIG. 10 is a continuation of FIG. 9 and

[0029]FIG. 11 shows cross-sectional views of the manufacturing processesof FIG. 9 and FIG. 10.

[0030]FIG. 12(a) is a cross-sectional view of a prior art bimorph typepiezoelectric element for acceleration sensor (cantilever structure) andFIG. 12(b) is a cross-sectional view of a bimorph type piezoelectricelement for acceleration sensor (two end supported beam structure) witha prior art structure.

[0031]FIG. 13 shows cross-sectional process views of a typical methodfor manufacturing a bimorph type piezoelectric element for accelerationsensor (cantilever structure) according to a prior art process.

[0032]FIG. 14 shows cross-sectional process views of a typical methodfor manufacturing a bimorph type piezoelectric element for accelerationsensor (two end supported beam structure) according to a prior artprocess.

PREFERRED EXEMPLARY EMBODIMENTS OF THE INVENTION First ExemplaryEmbodiment

[0033] A detailed explanation will be made on a bimorph typepiezoelectric element for acceleration sensor in a first exemplaryembodiment of the present invention with reference to FIG. 1. FIG. 1(a)is a cross-sectional view of a bimorph type piezoelectric element foracceleration sensor with a cantilever structure and FIG. 1(b) is aperspective view of the same. The reference symbol 32 a indicates afreely vibrating part that is formed by grinding away away and makes anacceleration detecting sensor, and the reference symbol 33 a indicates asupporting member formed at the same time of the part that was notground away when the freely vibrating part was formed, thus being madeintegral with the freely vibrating part 32 a. The freely vibrating part32 a was made to measure 0.1 mm in thickness, 2 mm in length L3 and 0.5mm in width. The thickness of the supporting member 33 a was made tomeasure 0.4 mm. The freely vibrating part 32 a is formed of twosingle-crystal plates of LiNbO₃(lithium niobate, referred to as LNhereafter), which are directly bonded together with no adhesive used insuch a way as the positively polarized surface of each respectivesingle-crystal plate forming one of the opposing main surfaces is puttogether with the other face to face.

[0034] The two LN single-crystal plates are made completely integralwith each other by direct bonding and the bonding surface, wherepolarization directions are reversed, is indicated by a dotted line A inFIG. 1.

[0035] Each of the two LN single-crystal plates that are directly bondedtogether with the polarization reversed boundary A made a boundarysurface is ground away to the same thickness of 0.05 mm. The detectionsensitivity S of the sensor is expressed by an inequality of:

Sα>d/ε*ρ*L1*α

[0036] where d: piezoelectric constant, ε: dielectric constant, ρ: massdensity, L1: length of free vibrating part, and β: constant. An LNsingle-crystal plate is an anisotropic material, resulting in adifferent piezoelectric constant d and a different dielectric constant εaccording to a cutting plane. Therefore, a 120° to 150° rotation Y plateis obtained in advance by a simulation to maximize d/ε, therebyrealizing high detection sensitivity. Furthermore, by having the bondingof the single-crystal plates performed so as to have the polarizationdirections reversed therebetween, generation of electrical chargeswithin the freely vibrating part 32 a due to a temperature rise iscanceled with a resulting realization of temperature characteristics forexcellent sensor sensitivity.

[0037] The reference symbols 31 a and 31 b indicate electrodes fortapping off electrical charges that are generated at the time ofacceleration detection. Cr/Au or Ti/Au and the like are desirable as thematerial for the electrodes in consideration of the adhesion strengthwith the underlying LN single-crystal plate and the stability of theelectrode film. When acceleration is applied to the whole assembly withthe supporting member 33 a fixed in position, a distortion is created inthe freely vibrating part 32 a and electrical charges generated inproportion to the distortion are tapped off through the electrodes 31 aand 31 b, thereb the applied acceleration being detected. Forming asupporting member integrally with a freely vibrating part 32 a to looklike a step and to form a cantilever structure as a whole without usingany adhesives and the like used and also selecting the cutting plane ofthe LN single-crystal plate so as to maximize the piezoelectric constantd and electromechanical coupling factor k, it was made possible to gainhigh sensitivity of 6 mV/G, which is two to three times the sensitivityof a bimorph type piezoelectric element for acceleration sensor usingceramic piezoelectric elements.

Second Exemplary Embodiment

[0038] A detailed explanation will be made on a bimorph typepiezoelectric element for acceleration sensor in a second exemplaryembodiment of the present invention with reference to FIG. 2. FIG. 2(a)is a cross-sectional view of a bimorph type piezoelectric element foracceleration sensor with a two end supported beam structure and FIG.2(b) is a perspective view of the same. The reference symbol 32 bindicates a freely vibrating part that is formed by grinding away andmakes an acceleration detecting sensor, and the reference symbols 33 band 33 c indicate supporting members formed, at the same time, of thepart that was not ground away when the freely vibrating part was formed,thus being made integral with the freely vibrating part 32 b. The freelyvibrating part 32 b was made to measure 0.1 mm in thickness, 2 mm inlength L4 and 0.5 mm in width. The thickness of each of the supportingmembers 33 b and 33 c was made to measure 0.4 mm. The freely vibratingpart 32 b is formed of two single-crystal plates of LN, which aredirectly bonded together with no adhesive used in such a way as thepositively polarized surface of each respective single-crystal plate,which forms an opposing main surface is put together with the other faceto face.

[0039] The two LN single-crystal plates are made completely integralwith each other by direct bonding and the bonding surface, wherepolarization directions are reversed, is indicated by a dotted line B inFIG. 2. Each of the two LN single-crystal plates that are directlybonded together with the polarization reversed boundary B made thethreshold therebetween is ground away to the same thickness of 0.05 mm.The detection sensitivity S of the sensor is expressed by an inequalityof:

Sα>d/ε*ρ*L1*α

[0040] , where d: piezoelectric constant, ε: dielectric constant, ρ:mass density, L1: length of free vibrating part, and β: constant. An LNsingle-crystal plate is an anisotropic material, resulting in adifferent piezoelectric constant d and a different dielectric constant εaccording to a cutting plane. Therefore, a 120° to 150° rotation Y plateis obtained in advance by a simulation to maximize d/ε, therebyrealizing high detection sensitivity. Furthermore, by having the bondingof the single-crystal plates performed so as to reverse the polarizationdirection, generation of electrical charges within the freely vibratingpart 32 b due to a temperature rise is canceled with a resultingrealization of temperature characteristics for excellent sensorsensitivity.

[0041] The reference symbols 31 c and 31 d indicate electrodes fortapping off electrical charges that are generated at the time ofacceleration detection. Cr/Au or Ti/Au and the like are desirable as thematerial for the electrodes in consideration of the adhesion strengthwith the underlying LN single-crystal plate and the stability of theelectrode film. When acceleration is applied to the whole assembly withthe supporting members 33 b and 33 c fixed in position, a distortion iscreated in the freely vibrating part 32 b and electrical chargesgenerated in proportion to the distortion are tapped off through theelectrodes 31 c and 31 d, thereby the applied acceleration beingdetected. Forming supporting members integrally with a freely vibratingpart 32 b with each respective supporting member looking like a step toform a two end supported beam structure as a whole without using anyadhesives and the like and also selecting the cutting plane of the LNsingle-crystal plate so as to maximize the piezoelectric constant d andelectromechanical coupling factor k, it was made possible to gain highsensitivity, which is two to three times the sensitivity of a bimorphtype piezoelectric element for acceleration sensor using ceramicpiezoelectric elements.

Third Exemplary Embodiment

[0042] A detailed explanation will be made on a method for manufacturinga bimorph type piezoelectric element for acceleration sensor in a thirdexemplary embodiment of the present invention with reference to FIG. 3to FIG. 5. FIG. 3 and FIG. 4 show perspective process views of theforegoing manufacturing method and FIG. 5 shows cross-sectional views ofthe manufacturing steps of FIG. 3 and FIG. 4. The reference symbols 6 eand 6 f indicate an LN single-crystal plate, respectively, and thereference symbols 5 a and 5 b indicate polarization directions of the LNsingle-crystal plates, respectively. The dotted lines in the drawingsindicate a polarization reversed boundary A, where two LN single-crystalplates are directly bonded together with the polarization directionsthereof reversed from each other. The reference symbol 7 c indicates afreely vibrating part (formed by grinding away) and the reference symbol8 c indicates a supporting member (prepared with no grinding awayapplied). The reference symbol 12 c indicates grindstones used forgrinding away and the reference symbol 13 c indicates spacers for fixingthe position of the grindstones 12 c with a predetermined spacingprovided therebetween. The reference symbol 9 indicates an electrodeformed on the main surface of the two IN single-crystal plates that havebeen directly bonded together and the reference symbols 17 a to 17 f and16 a to 16 p indicate cutting directions.

[0043] The method for manufacturing a bimorph type piezoelectric elementfor acceleration sensor in the present exemplified embodiment startswith putting together the two LN single-crystal plates 6 e and 6 f aftercleaning each respective surface thereof and then applying heat to thecombined two plates to directly bond the two plates together. (Steps ato b) The thickness of each respective LN single-crystal plate isdetermined in consideration of the final configuration. In this case,two LN single-crystal plates, each measuring 0.35 mm in thickness, forexample, are directly bonded together. The steps of cleaning and puttingtogether of the two plates are preferred to be performed in a clean roomin order to keep dust from entering between two opposing main surfacesof the plates, to be directly bonded together. In addition, the twoopposing main surfaces of the LN single-crystal plates are both madepositive in polarity. The detection sensitivity of the sensor isexpressed by an inequality of:

Sα>d/ε*ρ*L1*α

[0044] , where d: piezoelectric constant, ε: dielectric constant, ρ:mass density, L1: length of free vibrating part, and β: constant. An LNsingle-crystal plate is an anisotropic material, resulting in adifferent piezoelectric constant d and a different dielectric constant eaccording to a cutting plane. Therefore, a 120° to 150° rotation Y plateis obtained in advance by a simulation to maximize d/ε, therebyrealizing high detection sensitivity. Furthermore, by having the bondingof the single-crystal plates performed so as to have the two opposingmain surfaces made both positive in polarity, generation of electricalcharges due to a temperature rise is canceled with a resultingrealization of temperature characteristics for excellent sensorsensitivity. Additionally, by having the amount of displacement of eachrespective plate from the x axis or z axis limited within ±1°when thebonding is performed, the bonding strength can be enhanced, resulting inan improvement in the shock resistance of an acceleration sensor. As aresult of tensile tests, the temperature of the heat that is appliedafter the two plates are put together is made 275° C. or higher, where adestruction mode does not take place in the bonded boundary but bulkdestruction is caused to occur. The upper limit of the heatingtemperature can be extended to near 1150° C., which corresponds to theCurie temperature of LN. Costs of the equipment involved can be reducedby performing the heating in vacuum or in the atmosphere. Uponcompletion of the heating, the pair of LN single-crystal plates 6 e and6 f that have been bonded together are completely made integral witheach other although the polarity reversed boundary A forming a bondingplane is indicated by a dotted line in the drawings.

[0045] Next, one of the LN single-crystal plates directly bonded isscraped to an extent of 0.3 mm by means of a lapping method, a surfacegrinding away method or the like until the thickness t1 to the polarityreversed boundary reaches 0.05 mm. (Step c) If an LN single-crystalplate of 0.05 mm thick is used for the direct bonding, this Step c canbe omitted.

[0046] Then, the other surface of the LN single-crystal plate formed bydirect bonding is ground away by using the grindstone 12 c with athickness of W1 to produce the freely vibrating parts 7 c, each of whichmakes a sensor element and is made simultaneously and integral with thesupporting member 8 c that is prepared with no grinding away applied andintended for supporting a cantilever beam. (Step d) At least one or moreof the grindstone 12 c is used in grinding away and the grindstone'swidth W1 and the width W2 of a spacer for fixing grindstones aredetermined in consideration of the length of the freely vibrating part 7c and the length of the supporting member 8 c. In this case, W1 is made2 mm, W2 is made 0.5 mm, the length of the freely vibrating part 7 c ismade 2 mm and the length of the supporting member 8 c is made 0.5 mm. Bymaking the extent of grinding away from the surface measure 0.3 mm andthe thickness of the freely vibrating part 7 c measure 0.1 mm, thegrinding away is performed so as to make the thicknesses t2 and t3 ofrespective LN single-crystal plates, which are directly bonded eachother with the polarity reversed boundary A serving as the thresholdtherebetween, the same 0.05 mm. By making the thicknesses t2 and t3 thesame with each other, high sensitivity has been realized.

[0047] Next, electrodes 9 for detecting electrical charges are formed onboth the front and back surfaces of the LN single-crystal plate producedthrough a grinding away process. (Step e) In this case, the electrodes 9are formed by vapor deposition or electroless plating and it ispreferred to employ Cr/Au or Ti/Au as the electrode material inconsideration of the adhesion strength with the LN single-crystal plate.

[0048] Then, the LN single-crystal plate is cut in a matrix format bydicing or by the use of a slicer as indicated by the reference symbols17 a to 17 f and 16 a to 16 p to produce bimorph type piezoelectricelements for acceleration sensor, each having a cantilever beamstructure provided with a supporting member 8 c and a freely vibratingpart 7 c. (Step f) When the foregoing cutting process is performed, itis needed to use at least one or more of cutting edge and the spacingbetween the edges has to be determined in consideration of theconfiguration of the sensor to be produced. In this particular case, abimorph type piezoelectric element for acceleration sensor of anextremely small size of 2.5 mm long, 0.5 mm wide and 0.4 mm thick hasbeen realized by employing a spacing of 2.5 mm between the respectiveadjoining reference symbols of 17 a to 17 f and a spacing of 0.5 mmbetween the respective adjoining reference symbols of 16 a to 16 p.Since the length of the freely vibrating part 7 c is determinedaccording to the accuracy of grindstones 12 c that are employed and themachine accuracy that is applicable at the time of grinding away andcutting, a very good accuracy of ±3% has been achieved in the variationsin sensor sensitivity with the present invention in comparison with theconventional method of fixing the supporting member with the use ofadhesives, whereby the variations in sensor sensitivity have been madeas wide as ±20%. With respect to the average time required to fix asupporting member, it takes 5 minutes per a unit of the supportingmember according to the conventional method of bonding while taking only0.01 minutes with the present invention, which correspond to a greatreduction to {fraction (1/50)}of the time needed by the conventionalmethod of bonding (even when one grind stone is used in the grindingaway process). Thus, the present invention can provide a method formanufacturing a bimorph type piezoelectric element of cantilever beamstructure for acceleration sensor exhibiting a narrow range of variationin sensitivity and also achieving a low cost.

Fourth Exemplary Embodiment

[0049] A detailed explanation will be made on a method for manufacturinga bimorph type piezoelectric element for acceleration sensor in a fourthexemplary embodiment of the present invention with reference to FIG. 6to FIG. 8. FIG. 6 and FIG. 7 show perspective process views of theforegoing manufacturing method and FIG. 8 shows cross-sectional views ofthe manufacturing steps of FIG. 6 and FIG. 7. The reference symbols 6 cand 6 d indicate an LN single-crystal plate, respectively, and thereference symbols 5 a and 5 b indicate polarization directions of the LNsingle-crystal plates, respectively. The dotted lines in the drawingsindicate a polarization reversed boundary B, where two LN single-crystalplates are directly bonded together with the polarization directionsthereof reversed from each other. The reference symbol 7 b indicates afreely vibrating part (formed by grinding away) and the reference symbol8 b indicates a supporting member (prepared with no grinding awayapplied). The reference symbol 12 b indicates grindstones used forgrinding away and the reference symbol 13 b indicates spacers for fixingthe position of the grindstones 12 b with a predetermined spacingprovided therebetween. The reference symbol 9 indicates an electrodeformed on the main surface of the two LN single-crystal plates that havebeen directly bonded together and the reference symbols 14 a to 14 c and15 a to 15 k indicate cutting directions.

[0050] The method for manufacturing a bimorph type piezoelectric elementfor acceleration sensor in the present exemplified embodiment startswith putting together the two LN single-crystal plates 6 c and 6 d aftercleaning each respective surface thereof and then applying heat to thecombined two plates to directly bond the two plates together. (Steps ato b) The thickness of each respective LN single-crystal plate isdetermined in consideration of the final configuration. In this case,two LN single-crystal plates, each measuring 0.35 mm in thickness, forexample, are directly bonded together. The steps of cleaning and puttingtogether of the two plates are preferred to be performed in a clean roomin order to keep dust from entering between the two opposing mainsurfaces of the plates, which are directly bonded together. In addition,the two opposing main surfaces of the LN single-crystal plates are bothmade positive in polarity. The detection sensitivity of the sensor isexpressed by an inequality of:

Sα>d/ε*ρ*L1*α

[0051] , where d: piezoelectric constant, ε: dielectric constant, ρ:mass density, L1: length of free vibrating part, and β: constant. An LNsingle-crystal plate is an anisotropic material, resulting in adifferent piezoelectric constant d and a different dielectric constant Eaccording to a cutting plane. Therefore, a 120° to 150° rotation Y plateis obtained in advance by a simulation to maximize d/ε, therebyrealizing high detection sensitivity. Furthermore, by having the bondingof the single-crystal plates performed so as to have the two opposingmain surfaces made both positive in polarity, generation of electricalcharges due to a temperature rise is canceled with a resultingrealization of temperature characteristics for excellent sensorsensitivity. Additionally, by having the amount of displacement of eachrespective plate from the x axis or z axis limited within ±1° when thebonding is performed, the bonding strength can be enhanced, therebyobtaining an improvement in the shock resistance of a resultingacceleration sensor. As a result of tensile tests, the temperature ofthe heat that is applied after the two plates are put together is made275° C. or higher, where a destruction mode does not take place in thebonded boundary but bulk destruction is caused to occur. The upper limitof the heating temperature can be extended to near 1150° C., whichcorresponds to the Curie temperature of LN. Costs of the equipmentinvolved can be reduced by performing the heating in vacuum or in theatmosphere. Upon completion of the heating, the pair of LNsingle-crystal plates 6 c and 6 d that have been bonded together arecompletely made integral with each other although the polarity reversedboundary B forming a bonding plane is indicated by a dotted line in thedrawings.

[0052] Next, one of the surfaces of the LN single-crystal plate formedby direct bonding is scraped to an extent of 0.3 mm by means of alapping method, a surface grinding away method or the like until thethickness t1 from the polarity reversed boundary reaches 0.05 mm. (Stepc) If an LN single-crystal plate of 0.05 mm thick is used for the directbonding, this Step c can be omitted.

[0053] Then, the other surface of the IN single-crystal plate formed bydirect bonding is ground away by using the grindstone 12 c with athickness of W1 to produce the freely vibrating parts 7 c, each of whichmakes a sensor element and is made simultaneously and integral with thesupporting member 8 b that is prepared with no grinding away applied andintended for a supporting member with a two end supported beam. (Step d)At least one or more of the grindstone 12 b is used in grinding away andthe grindstone's width W1 and the width W2 of a spacer for fixinggrindstones are determined in consideration of the length of the freelyvibrating part 7 b and the length of the supporting member 8 b. In thiscase, W1 is made 2 mm, W2 is made 0.5 mm, the length of the freelyvibrating part 7 b is made 2 mm and the length of the supporting member8 b is made 0.5 mm. By making the extent of grinding away from thesurface measure 0.3 mm and the thickness of the freely vibrating part 7b measure 0.1 mm, the grinding away is performed so as to make thethicknesses t2 and t3 of respective LN single-crystal plates, which aredirectly bonded each other with the polarity reversed boundary B servingas the threshold therebetween, the same 0.05 mm. By making thethicknesses t2 and t3 the same with each other, high sensitivity hasbeen realized.

[0054] Next, electrodes 9 for detecting electrical charges are formed onboth the front and back surfaces of the LN single-crystal plate producedthrough a grinding away process. (Step e) In this case, the electrodes 9are formed by vapor deposition or electroless plating and it ispreferred to employ Cr/Au or Ti/Au as the electrode material inconsideration of the adhesion strength with the LN single-crystal plate.

[0055] Then, the LN single-crystal plate is cut in a matrix format bydicing or by the use of a slicer as indicated by the reference symbols14 a to 14 c and 15 a to 15 k to produce bimorph type piezoelectricelements for acceleration sensor, each having a two end supported beamstructure provided with a supporting member 8 b and a freely vibratingpart 7 b. (Step f) When the foregoing cutting process is performed, itis needed to use at least one or more of cutting edge and the spacingbetween the edges has to be determined in consideration of theconfiguration of the sensor to be produced. In this particular case, abimorph type piezoelectric element for acceleration sensor of anextremely small size of 2.5 mm long, 0.5 mm wide and 0.4 mm thick hasbeen realized by employing a spacing of 2.5 mm between the respectiveadjoining reference symbols of 14 a to 14 c and a spacing of 0.5 mmbetween the respective adjoining reference symbols of 15 a to 15 k.Since the length of the freely vibrating part 7 b is determinedaccording to the accuracy of grindstones 12 b that are employed and alsothe machine accuracy that is applicable at the time of grinding away andcutting, a very good accuracy of ±3% has been achieved in the variationsin sensor sensitivity with the present invention in comparison with theconventional method of fixing the supporting member with the use ofadhesives, whereby the variations in sensor sensitivity have been madeas wide as ±20%. With respect to the average time required to fix asupporting member, it takes 5 minutes per a unit of the supportingmember according to the conventional method of bonding while taking only0.01 minutes with the present invention, which correspond to a greatreduction to {fraction (1/50)}of the time needed by the conventionalmethod of bonding (even when one grind stone is used in the grindingaway process). Thus, the present invention can provide a method formanufacturing a bimorph type piezoelectric element of two end supportedbeam structure for acceleration sensor exhibiting a narrow range ofvariation in sensitivity and also achieving a low cost.

Fifth Exemplary Embodiment

[0056] A detailed explanation will be made on a method for manufacturinga bimorph type piezoelectric element for acceleration sensor in a fifthexemplary embodiment of the present invention with reference to FIG. 9to FIG. 11. FIG. 9 and FIG. 10 show perspective process views of theforegoing manufacturing method and FIG. 11 shows cross-sectional viewsof the manufacturing steps of FIG. 9 and FIG. 10. The reference symbols6 a and 6 b indicate an LN single-crystal plate, respectively, and thereference symbols 5 a and 5 b indicate polarization directions of the LNsingle-crystal plates, respectively. The dotted lines in the drawingsindicate a polarization reversed booundary C, where two LNsingle-crystal plates are directly bonded together with the polarizationdirections thereof reversed from each other. The reference symbol 7 aindicates a freely vibrating part (formed by grinding away) and thereference symbol 8 a indicates a supporting member (prepared with nogrinding away applied). The reference symbol 12 a indicates grindstonesused for grinding away and the reference symbol 13 a indicates spacersfor fixing the position of the grindstones 12 a with a predeterminedspacing provided therebetween. The reference symbol 9 indicates anelectrode formed on the main surface of the two LN single-crystal platesthat have been directly bonded together and the reference symbols 10 ato 10 f and 11 a to 11 k indicate cutting directions.

[0057] The method for manufacturing a bimorph type piezoelectric elementfor acceleration sensor in the present exemplified embodiment startswith putting together the two LN single-crystal plates 6 a and 6 b aftercleaning each respective surface thereof and then applying heat to thecombined two plates to directly bond the two plates together (Steps a tob). The thickness of each respective LN single-crystal plate isdetermined in consideration of the final configuration. In this case,two LN single-crystal plates, each measuring 0.35 mm in thickness, forexample, are directly bonded together. The steps of cleaning and puttingtogether of the two plates are preferred to be performed in a clean roomin order to keep dust from entering between two opposing main surfacesof the plates, which are directly bonded together. In addition, the twoopposing main surfaces of the LN single-crystal plates are both madepositive in polarity. The detection sensitivity of the sensor isexpressed by an inequality of:

Sα>d/ε*ρ*L1*α

[0058] , where d: piezoelectric constant, ε: dielectric constant, ρ:mass density, L1: length of free vibrating part, and β: constant. An LNsingle-crystal plate is an anisotropic material, resulting in adifferent piezoelectric constant d and a different dielectric constant Eaccording to a cutting plane. Therefore, a 120° to 150° rotation Y plateis obtained in advance by a simulation to maximize d/ε, therebyrealizing high detection sensitivity. Furthermore, by having the bondingof the single-crystal plates performed so as to have the two opposingmain surfaces made both positive in polarity, generation of electricalcharges due to a temperature rise is canceled with a resultingrealization of temperature characteristics for excellent sensorsensitivity. Additionally, by having the amount of displacement of eachrespective plate from the x axis or z axis limited within ±1° when thebonding is performed, the bonding strength can be enhanced, therebyobtaining an improvement in the shock resistance of a resultingacceleration sensor. As a result of tensile tests, the temperature ofthe heat that is applied after the two plates are put together is made275° C. or higher, where a destruction mode does not take place in thebonded boundary but bulk destruction is caused to occur. The upper limitof the heating temperature can be extended to near 1150° C., whichcorresponds to the Curie temperature of LN. Costs of the equipmentinvolved can be reduced by performing the heating in vacuum or in theatmosphere. Upon completion of the heating, the pair of LNsingle-crystal plates 6 a and 6 b that have been bonded together arecompletely made integral with each other although the polarity reversedboundary C forming a bonding plane is indicated by a dotted line in thedrawings.

[0059] Next, one of the surfaces of the LN single-crystal plate formedby direct bonding is scraped to an extent of 0.3 mm by means of alapping method, a surface grinding away method or the like until thethickness t1 from the polarity reversed boundary reaches 0.05 mm. (Stepc) If an LN single-crystal plate of 0.05 mm thick is used for the directbonding, this Step c can be omitted.

[0060] Then, the other surface of the IN single-crystal plate formed bydirect bonding is ground away by using the grindstone 12 a with athickness of W1 to produce the freely vibrating parts 7 a, each of whichmakes a sensor element and is made simultaneously and integral with thesupporting member 8 a that is prepared with no grinding away applied andintended for a supporting member of a cantilever beam. (Step d) At leastone or more of the grindstone 13 a is used in grinding away and thegrindstone's width W1 and the width W2 of a spacer for fixinggrindstones are determined in consideration of the length of the freelyvibrating part 7 a and the length of the supporting member 8 a. In thiscase, W1 is made 4.0 mm, W2 is made 1.0 mm, the length of the freelyvibrating part 7 a is made 4.0 mm and the length of the supportingmember 8 a is made 1.0 mm. By making the extent of grinding away fromthe surface measure 0.3 mm and the thickness of the freely vibratingpart 7 a measure 0.1 mm, the grinding away is performed so as to makethe thicknesses t2 and t3 of respective LN single-crystal plates, whichare directly bonded each other with the polarity reversed boundary Cserving as the threshold therebetween, the same 0.05 mm. By making thethicknesses t2 and t3 the same with each other, high sensitivity hasbeen realized.

[0061] Next, electrodes 9 for detecting electrical charges are formed onboth the front and back surfaces of the LN single-crystal plate producedthrough a grinding away process. (Step e) In this case, the electrodes 9are formed by vapor deposition or electroless plating and it ispreferred to employ Cr/Au or Ti/Au as the electrode material inconsideration of the adhesion strength with the LN single-crystal plate.

[0062] Then, the LN single-crystal plate is cut in a matrix format bydicing or by the use of a slicer as indicated by the reference symbols10 a to 10 f and 11 a to 11 k to produce bimorph type piezoelectricelements for acceleration sensor, each having a cantilever beamstructure provided with a supporting member 8 a and a freely vibratingpart 7 a. (Step f) When the foregoing cutting process is performed, itis needed to use at least one or more of cutting edge and the spacingbetween the edges has to be determined in consideration of theconfiguration of the sensor to be produced. In this particular case, abimorph type piezoelectric element for acceleration sensor of anextremely small size of 2.5 mm long, 0.5 mm wide and 0.4 mm thick hasbeen realized by employing a spacing of 2.5 mm between the respectiveadjoining reference symbols of 10 a to 10 f and a spacing of 0.5 mmbetween the respective adjoining reference symbols of 11 a to 11 k.Since the length of the freely vibrating part 7 a is determinedaccording to the accuracy of grindstones 12 a that are employed and alsothe machine accuracy that is applicable at the time of grinding away andcutting, a very good accuracy of ±3% has been achieved in the variationsin sensor sensitivity with the present invention in comparison with theconventional method of fixing the supporting member with the use ofadhesives, whereby the variations in sensor sensitivity have been madeas wide as ±20%. By employing grindstones 12 a for the freely vibratingparts 7 a, each measuring 4 mm thick that corresponds to two times thethickness as used in the Third Exemplary Embodiment, the time requiredin the grinding away process is reduced to one half of that in the ThirdExemplary Embodiment. With respect to the average time required toproduce a supporting member, it takes 0.005 minutes per a unit I, of thesupporting member according to the present invention, which correspondto a great reduction to {fraction (1/100)}of the time needed by theconventional method (even when one i grind stone is used in the grindingaway process). Thus, the present invention can provide a method formanufacturing a bimorph type piezoelectric element of cantilever V beamstructure for acceleration sensor exhibiting a narrow range of variationin sensitivity and also achieving a low cost.

[0063] Industrial Applicability

[0064] As described in the above, the present invention relates to abimorph type piezoelectric element for acceleration sensor formed of apair of piezoelectric single-crystal plates put together face to face bydirect bonding and characterized by having a structure with a freelyvibrating part formed on at least one of the foregoing pair ofpiezoelectric single-crystal plates by grinding away partially and alsoa supporting member or supporting members formed of the part that is notapplied with a grinding away process and made integral with theforegoing freely vibrating part at one end or both ends thereof, and amanufacturing method that includes:

[0065] a first step of direct bonding, whereby the foregoing pair ofpiezoelectric single-crystal plates are put together face to face andheated;

[0066] a second step of forming a freely vibrating part by grinding awayat least one of the directly bonded pair of piezoelectric single-crystalplates in the direction of rows or columns at a predetermined spacing toa predetermined depth;

[0067] a third step of forming electrodes on the ground away mainsurface of the foregoing piezoelectric single-crystal plate; and afourth step of forming supporting members by cutting both the groundaway freely vibrating part and the part that is not applied with agrinding away process at a predetermined spacing in the directions ofboth rows and columns.

[0068] Accordingly, a bimorph type piezoelectric element foracceleration sensor that is small in size, high in sensitivity andreduced in variations in sensitivity can be produced at a low cost.

1. A bimorph type piezoelectric element for acceleration sensor formed of a pair of piezoelectric single-crystal plates put together face to face by direct bonding comprising: a freely vibrating part formed on at least one of said pair of piezoelectric single-crystal plates by grinding away partially; and a supporting member or supporting members formed of the part that is not applied with a grinding away process and made integral with said freely vibrating part at one end or both ends thereof.
 2. The bimorph type piezoelectric element for acceleration sensor according to claim 1 , wherein said piezoelectric single-crystal plate is formed of lithium niobate single crystals and the opposing main surface thereof is made as a cutting out plane of 120° to 150° rotation Y plate.
 3. The bimorph type piezoelectric element for acceleration sensor according to claim 1 , wherein the opposing main surface of said piezoelectric single-crystal plate is a positively polarized surface.
 4. The bimorph type piezoelectric element for acceleration sensor according to claim 1 , wherein each respective plate of said pair of single-crystal plates directly bonded together and forming said freely vibrating part has the same thickness with each other.
 5. A method for manufacturing a bimorph type piezoelectric element for acceleration sensor comprising: a first step of directly bonding a pair of piezoelectric single-crystal plates face to face and applying heat thereto; a second step of forming a freely vibrating part by grinding away at least one of said directly bonded pair of piezoelectric single-crystal plates in the direction of rows or in the direction of columns at a predetermined spacing to a predetermined depth; a third step of forming electrodes on the ground away main surface of said piezoelectric single-crystal plate; and a fourth step of forming supporting members by cutting both said ground away freely vibrating part and said part that is not applied with a grinding away process at a predetermined spacing in the directions of both rows and columns.
 6. The method for manufacturing a bimorph type piezoelectric element for acceleration sensor according to claim 5 , wherein positively polarized surfaces of said pair of piezoelectric single-crystal plates are directly bonded together.
 7. The method for manufacturing a bimorph type piezoelectric element for acceleration sensor according to claim 5 , wherein the temperature of the heat applied to said pair of piezoelectric single-crystal plates after being put together face to face is 275° C or higher.
 8. The method for manufacturing a bimorph type piezoelectric element for acceleration sensor according to claim 5 , wherein said freely vibrating part is formed by grinding away said pair of piezoelectric single-crystal plates formed of two opposing plates provided with the same thickness with each other.
 9. The method for manufacturing a bimorph type piezoelectric element for acceleration sensor according to claim 5 , wherein said electrodes on the ground away surfaces of said piezoelectric single-crystal plate are formed by vapor deposition or electroless plating.
 10. The method for manufacturing a bimorph type piezoelectric element for acceleration sensor according to claim 5 , wherein a cutting means is used in said fourth step and said cutting means comprises at least one or more of cutting edge fixed on the same axis at a predetermined spacing.
 11. The method for manufacturing a bimorph type piezoelectric element for acceleration sensor according to claim 5 , wherein a grinding away means used in said second step comprises at least one or more of grindstone fixed on the same axis. 